Sulfur-containing gas streams in petroleum refineries and natural gas plants are typically desulfurized by the Claus process. The Claus process operates in two major process steps. In the first, hydrogen sulfide is converted to elemental sulfur at temperatures of approximately 1000° C. by the combustion of approximately one-third of the H2S in the gas stream to produce sulfur dioxide which then reacts with the remaining H2S to produce elemental sulfur. Following condensation and removal of the molten sulfur formed in this stage, the reaction between the H2S and the SO2 is continued in the second, catalytic step in which elemental sulfur is produced at temperatures between 200-350° C. over an alumina catalyst. The Claus reaction can be represented by the equations:H2S+1.5O2→SO2+H2O2H2S+SO2→3S+2H2O
As the second reaction is an equilibrium reaction which is favored at lower temperatures, it is carried out in stages with condensation and removal of molten elemental sulfur between each stage, followed by heating to the reaction temperature for the next stage. Typically, there are three stages of catalytic conversions although two stages are also conventional when a tail gas treatment unit is used. With the progressive stagewise removal of the sulfur between stages, the temperature of each stage is reduced to obtain the more favorable thermodynamic equilibrium: typically, the first catalytic stage will be operated at a temperature from 315 to 330° C., the second at about 240° C. and the third, if used, at around 200° C. with the outlet of each stage being maintained at least 20° C. above the dew point of the sulfur to avoid the generation of liquid within the catalyst beds. Operation of the first stage at high temperature ensures hydrolysis of a major portion of COS and CS2; operation of each bed close to the dew point of the sulfur brings conversion closer to the equilibrium value.
The Claus process has been improved over the years mainly by improvements intended to reduce the residual sulfur levels in the tail gas. The basic Claus process will generally produce an overall recovery of 95-97% sulfur but this is no longer considered adequate in most instances, mainly for environmental reasons. The Jacobs COMPRIMO™ SUPERCLAUS™ process, using a special catalyst in the last reactor oxidizes the H2S selectively to sulfur with air injected into the reactor to avoid formation of SO2, has a sulfur efficiency of around 99.0%, depending on the composition of the Claus feed.
The tail gas from the Claus unit contains residual quantities of sulfur in the form of elemental sulfur, sulfur dioxide, hydrogen sulfide as well as other sulfur-containing compounds such as COS and CS2 which will need to be removed if the highest degree of sulfur recovery is to be attained. In the United States, for example, a minimum sulfur recovery efficiency of 99.8% is required for larger Sulfur Recovery Units (SRUs), so that a tail gas treating unit is required.
Emissions from the Claus process may be reduced by: (1) extending the Claus reaction into a lower temperature liquid phase, (2) adding a scrubbing process to the Claus exhaust stream, or (3) incinerating the hydrogen sulfide gases to form sulfur dioxide. Processes currently available that extend the Claus reaction into a lower temperature liquid phase include Beavon Sulfur Recovery (BSR), BSR/Selectox, Sulfreen, Cold Bed Absorption, Maxisulf, and IFP-1 processes. All of these processes give higher overall sulfur recoveries of 98 to 99 percent when following downstream of a typical 2- or 3-stage Claus sulfur recovery unit.
Sulfur emissions can also be reduced by adding a scrubber at the tail end of the plant, these falling into one of two categories: oxidation tailgas scrubbers such as the Wellman-Lord, Stauffer Aquaclaus, and IFP-2 processes, and reduction tailgas scrubbers. In the reductive type of scrubbing process, sulfur in the tailgas is converted to H2S by hydrogenation in a catalytic reduction step after which the cooled tailgas is sent to a scrubber for H2S removal. Processes of this type include the Beavon and SCOT (Shell Claus Off-gas Treating) processes with the SCOT process being the current market leader in Claus tail gas treatment in spite the high capital requirement of the SCOT process, approximately 30 to 50 percent of the cost of the Claus plant itself.
The Beavon process (BSR), described originally in U.S. Pat. No. 3,752,877, heats the Claus tail gas typically to 290-340° C. by combustion of natural gas in an on-line Reducing Gas Generator (RGG) for subsequent catalytic reduction of virtually all non-H2S sulfur components to H2S which is then removed by amine scrubbing. Conversion of SO2 and elemental sulfur is by hydrogenation, while CO, COS and CS2 are hydrolyzed.
Various acid gas scrubbing processes are available for H2S removal. These processes generally use an amine solution to remove the H2S and possibly other acid gas contaminants by reaction with the amines, after which the amines are regenerated in a separate column and the resulting H2S returned to the Claus plant feed. The Cansolv process removes sulfur dioxide in a similar manner and recycles regenerated sulfur dioxide to the Claus plant. Among the amine scrubbing processes is the highly effective FLEXSORB™ process, originally developed by Exxon Research and Engineering with its variants, the FLEXSORB SE™ and FLEXSORB SE Plus™ processes using proprietary severely hindered ethanolamine solvents which remove H2S to levels which are fully compliant with current regulatory requirements.
In summary, therefore, the SuperClaus process is the market-leading technology for 99.0-99.2% recovery. Currently the European Union BREF (Best Available Technology Reference) guidance to refiners is 99.5% sulfur recovery and for this intermediate recovery level, Lurgi's Sulfreen™ technology has been used. There is, however, a significant likelihood that this guidance will be upgraded to a requirement in coming years in view of current and pending legislation in various countries for enforcing 99.5% as a target sulfur recovery level. This exceeds the capabilities of the direct oxidation technology while the cost of a reduction-absorption-recycle unit such as SCOT for 99.5% recovery is prohibitively high. Longer term, it is expected that environmental legislation will continue to drive up sulfur recovery requirements. For 99.9+% recovery, ExxonMobili's FLEXSORB™ SE/SE Plus technology is the market leader. In the light of these expectations, the Sulfreen process, like the SuperClaus technology, is likely to become a regretted investment when the lower sulfur emissions/higher overall sulfur recovery required by future regulation are enforced. Thus refiners face the possibility of “regretted investment” should they choose a sulfur plant tail gas treating technology that can meet current requirements but not easily be upgraded to meet future specifications.
We have now devised a process which utilizes the unique capabilities of the sterically hindered aminoethoxyether sorbents to eliminate the reduction step in the SCOT and similar processes. The resulting process can more economically achieve the intermediate target of 99.5%-99.8% overall recovery and provide a phased-investment strategy for subsequent upgrade to achieve higher recoveries while still maintaining amine consumption in the scrubbing process at an acceptably low level.